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           #include <sys/epoll.h>


           The  epoll  API performs a similar task to poll(2): monitoring multiple
           file descriptors to see if I/O is possible on any of them.   The  epoll
           API can be used either as an edge-triggered or a level-triggered inter-
           face and scales well to large numbers of watched file descriptors.  The
           following  system  calls  are  provided  to  create and manage an epoll
           *  epoll_create(2)  creates  an  epoll  instance  and  returns  a  file
              descriptor  referring to that instance.  (The more recent epoll_cre-
              ate1(2) extends the functionality of epoll_create(2).)
           *  Interest in particular  file  descriptors  is  then  registered  via
              epoll_ctl(2).   The  set of file descriptors currently registered on
              an epoll instance is sometimes called an epoll set.
           *  epoll_wait(2) waits for I/O events, blocking the calling  thread  if
              no events are currently available.
       Level-triggered and edge-triggered
           The  epoll event distribution interface is able to behave both as edge-
           triggered (ET) and as level-triggered (LT).  The difference between the
           two mechanisms can be described as follows.  Suppose that this scenario
           1. The file descriptor that represents the read side of a pipe (rfd) is
              registered on the epoll instance.
           2. A pipe writer writes 2 kB of data on the write side of the pipe.
           3. A call to epoll_wait(2) is done that will return rfd as a ready file
           4. The pipe reader reads 1 kB of data from rfd.
           5. A call to epoll_wait(2) is done.
           If the rfd file descriptor has been added to the epoll interface  using
           the  EPOLLET  (edge-triggered)  flag, the call to epoll_wait(2) done in
           step 5 will probably hang despite the available data still  present  in
           the  file  input buffer; meanwhile the remote peer might be expecting a
           response based on the data it already sent.  The  reason  for  this  is
           that edge-triggered mode delivers events only when changes occur on the
           monitored file descriptor.  So, in step 5 the caller might end up wait-
           ing  for some data that is already present inside the input buffer.  In
           the above example, an event on rfd will be  generated  because  of  the
           write  done in 2 and the event is consumed in 3.  Since the read opera-
           tion done in 4 does not consume the whole  buffer  data,  the  call  to
           epoll_wait(2) done in step 5 might block indefinitely.
           Since even with edge-triggered epoll, multiple events can be  generated
           upon  receipt  of multiple chunks of data, the caller has the option to
           specify the EPOLLONESHOT flag, to tell epoll to disable the  associated
           file descriptor after the receipt of an event with epoll_wait(2).  When
           the EPOLLONESHOT flag is specified, it is the  caller's  responsibility
           to rearm the file descriptor using epoll_ctl(2) with EPOLL_CTL_MOD.
       /proc interfaces
           The following interfaces can be used to limit the amount of kernel mem-
           ory consumed by epoll:
           /proc/sys/fs/epoll/max_user_watches (since Linux 2.6.28)
                  This specifies a limit on the total number of  file  descriptors
                  that  a user can register across all epoll instances on the sys-
                  tem.  The limit is per  real  user  ID.   Each  registered  file
                  descriptor  costs  roughly  90  bytes  on  a  32-bit kernel, and
                  roughly 160 bytes on a 64-bit kernel.   Currently,  the  default
                  value  for  max_user_watches  is  1/25 (4%) of the available low
                  memory, divided by the registration cost in bytes.
       Example for suggested usage
           While the usage of epoll when employed as a  level-triggered  interface
           does  have  the  same  semantics  as  poll(2), the edge-triggered usage
           requires more clarification to avoid stalls in  the  application  event
           loop.   In this example, listener is a nonblocking socket on which lis-
           ten(2) has been called.  The function do_use_fd() uses  the  new  ready
           file descriptor until EAGAIN is returned by either read(2) or write(2).
           An event-driven state machine application should, after having received
           EAGAIN,  record  its  current  state  so  that  at  the  next  call  to
           do_use_fd() it will continue to  read(2)  or  write(2)  from  where  it
           stopped before.
               #define MAX_EVENTS 10
               struct epoll_event ev, events[MAX_EVENTS];
               int listen_sock, conn_sock, nfds, epollfd;
               /* Set up listening socket, 'listen_sock' (socket(),
                  bind(), listen()) */
               epollfd = epoll_create(10);
               if (epollfd == -1) {
      = EPOLLIN;
      = listen_sock;
               if (epoll_ctl(epollfd, EPOLL_CTL_ADD, listen_sock, &ev) == -1) {
                   perror("epoll_ctl: listen_sock");
                  = EPOLLIN | EPOLLET;
                  = conn_sock;
                           if (epoll_ctl(epollfd, EPOLL_CTL_ADD, conn_sock,
                                       &ev) == -1) {
                               perror("epoll_ctl: conn_sock");
                       } else {
           When  used  as an edge-triggered interface, for performance reasons, it
           is possible to add the  file  descriptor  inside  the  epoll  interface
           (EPOLL_CTL_ADD) once by specifying (EPOLLIN|EPOLLOUT).  This allows you
           to avoid continuously switching between EPOLLIN  and  EPOLLOUT  calling
           epoll_ctl(2) with EPOLL_CTL_MOD.
       Questions and answers
           Q0  What is the key used to distinguish the file descriptors registered
               in an epoll set?
           A0  The key is the combination of the file descriptor  number  and  the
               open  file  description  (also  known as an "open file handle", the
               kernel's internal representation of an open file).
           Q1  What happens if you register the same file descriptor on  an  epoll
               instance twice?
           A1  You  will  probably  get  EEXIST.  However, it is possible to add a
               duplicate (dup(2), dup2(2), fcntl(2)  F_DUPFD)  descriptor  to  the
               same  epoll instance.  This can be a useful technique for filtering
               events, if the duplicate file descriptors are registered with  dif-
               ferent events masks.
           Q2  Can  two epoll instances wait for the same file descriptor?  If so,
               are events reported to both epoll file descriptors?
           A2  Yes, and events would be reported to both.  However,  careful  pro-
               gramming may be needed to do this correctly.
           Q3  Is the epoll file descriptor itself poll/epoll/selectable?
           A3  Yes.   If  an epoll file descriptor has events waiting then it will
               indicate as being readable.
           Q4  What happens if one attempts to put an epoll file  descriptor  into
           A6  Yes, but be aware of the following point.  A file descriptor  is  a
               reference  to  an  open file description (see open(2)).  Whenever a
               descriptor is duplicated via dup(2), dup2(2), fcntl(2) F_DUPFD,  or
               fork(2),  a  new  file  descriptor  referring to the same open file
               description is created.  An  open  file  description  continues  to
               exist  until all file descriptors referring to it have been closed.
               A file descriptor is removed from an epoll set only after  all  the
               file  descriptors referring to the underlying open file description
               have been closed (or before if the descriptor is explicitly removed
               using  epoll_ctl(2)  EPOLL_CTL_DEL).   This means that even after a
               file descriptor that is part of  an  epoll  set  has  been  closed,
               events  may  be  reported  for  that  file descriptor if other file
               descriptors referring  to  the  same  underlying  file  description
               remain open.
           Q7  If more than one event occurs between epoll_wait(2) calls, are they
               combined or reported separately?
           A7  They will be combined.
           Q8  Does an operation on a file descriptor affect the already collected
               but not yet reported events?
           A8  You  can  do two operations on an existing file descriptor.  Remove
               would be meaningless for this case.  Modify will  reread  available
           Q9  Do I need to continuously read/write a file descriptor until EAGAIN
               when using the EPOLLET flag (edge-triggered behavior) ?
           A9  Receiving an event from epoll_wait(2) should suggest  to  you  that
               such file descriptor is ready for the requested I/O operation.  You
               must consider it ready  until  the  next  (nonblocking)  read/write
               yields  EAGAIN.   When  and how you will use the file descriptor is
               entirely up to you.
               For packet/token-oriented files (e.g., datagram socket, terminal in
               canonical  mode),  the only way to detect the end of the read/write
               I/O space is to continue to read/write until EAGAIN.
               For stream-oriented files (e.g., pipe, FIFO,  stream  socket),  the
               condition  that  the  read/write I/O space is exhausted can also be
               detected by checking the amount of data read from / written to  the
               target file descriptor.  For example, if you call read(2) by asking
               to read a certain amount of data and read(2) returns a lower number
               of  bytes,  you  can be sure of having exhausted the read I/O space
               for the file descriptor.  The  same  is  true  when  writing  using
               write(2).   (Avoid  this  latter  technique if you cannot guarantee
               that the monitored file descriptor always refers to  a  stream-ori-
               ented file.)
           o If using an event cache...
           If you use an event cache or store all the  file  descriptors  returned
           from epoll_wait(2), then make sure to provide a way to mark its closure
           dynamically (i.e., caused by a previous event's  processing).   Suppose
           you receive 100 events from epoll_wait(2), and in event #47 a condition
           causes event #13 to  be  closed.   If  you  remove  the  structure  and
           close(2) the file descriptor for event #13, then your event cache might
           still say there are events waiting for  that  file  descriptor  causing
           One  solution  for  this is to call, during the processing of event 47,
           epoll_ctl(EPOLL_CTL_DEL) to delete file  descriptor  13  and  close(2),
           then  mark  its  associated  data structure as removed and link it to a
           cleanup list.  If you find another event for file descriptor 13 in your
           batch processing, you will discover the file descriptor had been previ-
           ously removed and there will be no confusion.


           The epoll API was introduced in Linux kernel 2.5.44.  Support was added
           to glibc in version 2.3.2.


           The  epoll  API  is Linux-specific.  Some other systems provide similar
           mechanisms, for example, FreeBSD has kqueue, and Solaris has /dev/poll.


           epoll_create(2), epoll_create1(2), epoll_ctl(2), epoll_wait(2)

    Linux 2012-04-17 EPOLL(7)


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